Monolithic quantum cascade laser (QCL)/avalanche photodiode (APD) infrared transceiver

Abstract

A monolithic QCL/APD IR Transceiver in which the QCL transmitter and APD receiver have the same N MQW stage composition and variation in thickness in the z direction for all positions in x and y directions. The heterostructure is configured via asymmetric stages, additional stages for the APD or by reversing the polarity of the p-n junction for the APD or a combination thereof such that the upper energy state in the QCL under forward bias is confined to the quantum well and in the APD under reverse bias is near the top of the quantum well in energy and localized in the quantum well to spatially overlap with the lower energy state to facilitate detection of echo photons. The QCL and APD may be positioned end-to-end, side-by-side or as a common region of the heterostructure.

We claim: 
     
       1. A monolithic infrared transceiver, comprising:
 a monolithic intrinsic (i) semiconductor heterostructure including N Multiple Quantum Well (MQW) stages; 
 a first region of the heterostructure sandwiched between first and second mirrors oriented in a z-direction and separated in an x-direction to form a laser cavity and an n type doping layer above the laser cavity to inject electrons into a first MQW stage in which the heterostructure defines at least upper and lower energy states and a quantum well in each stage of a first quantum cascade laser (QCL), said first region responsive to application of a forward bias such that the upper and lower energy states in each MQW stage are confined in the quantum wells to induce intersubband lasing transitions in each of the MQW stages that contribute to a photon gain of the cavity to emit photons at a first IR wavelength in a positive x direction in a transmit mode; and 
 a second region of the heterostructure sandwiched between second p and n type doping layers in the z-direction that form a p-i-n semiconductor in which the heterostructure defines at least upper and lower energy states and a quantum well in each stage of an avalanche photodiode (APD), 
 wherein said first and second regions have the same N MQW stage composition and variation in thickness in the z direction for all positions in x and y directions; 
 wherein said monolithic intrinsic heterostructure is configured such that a separation energy between the upper and lower energy states in the first region under forward bias equals or slightly exceeds a separation energy between the upper and lower energy states in the second region under reverse bias by a delta separation energy such that under reverse bias in each stage of the second region the lower energy state is confined to the quantum well and the upper energy state is near the top of the quantum well in energy and localized in the quantum well to spatially overlap with the lower energy state such that the upper energy states moves into an avalanche multiplication region of the API) to detect echo photons at the first IR wavelength incident in the negative x direction in a receive mode. 
 
     
     
       2. The monolithic infrared transceiver of  claim 1 , wherein in each stage of the second region under reverse bias the upper energy state is either confined just below the top of the quantum well or lies just above the quantum well in the continuum states. 
     
     
       3. The monolithic infrared transceiver of  claim 1 , in which the heterostructure is a three-state system in which each stage includes the lower (E1), a middle (E2) and the upper (E3) states, wherein the forward biased first region intersubband lasing transition occurs from state E3→E2 while the E2→E1 transition is tuned to a phonon energy to depopulate state E2 so that E2 can accept an electron from the E3→E2 transition, wherein to detect the E3→E2 energy that was transmitted in an echo photon, under reverse bias of the second region the new energy positions of E1, E2 and E3 move such that absorption of the echo photon puts an electron in an uppermost state E3 near the top of the quantum well such that an echo photon is absorbed and promotes an electron to the upper energy state E3 where the electron is injected into the APD's reverse bias depletion field and is detected. 
     
     
       4. The monolithic infrared transceiver of  claim 1 , wherein each MQW stage comprises alternating quantum well and thin barrier layers in which the thickness of the quantum well layers varies such that each MQW state is asymmetric about a center position of the stage in the z direction. 
     
     
       5. The monolithic infrared transceiver of  claim 1 , wherein the monolithic intrinsic heterostructure includes an additional M MOW stages between the second p and n type doping layers in the z-direction in the second region to reduce a voltage drop across each of N+M MQW stages in the second region to provide the delta separation energy. 
     
     
       6. The monolithic infrared transceiver of  claim 1 , wherein the monolithic intrinsic heterostructure is configured with the first n type doping layer in the first region and the second n type doping layer in the second region on opposite sides of the heterostructure such that the APD has a reverse polarity with respect to the QCL to provide the delta separation energy. 
     
     
       7. The monolithic infrared transceiver of  claim 1 ,
 wherein said first and second regions coexist end-to-end in a single waveguide in the x direction at the same position in a y direction, 
 wherein in said receive mode, 
 the forward bias applied to the first region is reduced such that the QCL is in a state of population inversion and gain but below the threshold for lasing to allow echo photons to pass through the first region, and 
 the first minor is switchable to allow the echo photons to pass and enter the second region where the echo photon is detected by the reverse biased APD. 
 
     
     
       8. The monolithic infrared transceiver of  claim 7 , wherein said switchable first mirror comprises,
 top and bottom first metal mirror contacts between the first and second regions that sandwich a first mirror region of the heterostructure to form a m-i-m semiconductor; 
 wherein in said transmit mode, said m-i-m semiconductor responsive to a bias to change a refractive index of the heterostructure to insert the first mirror to form the laser cavity, and 
 wherein in said receive mode, said m-i-m semiconductor responsive to a bias to change a refractive index of the heterostructure to remove the first mirror and allow echo photons to enter the second region. 
 
     
     
       9. The monolithic infrared transceiver of  claim 1 , wherein said first and second regions exist in side-by-side first and second waveguides oriented in the x direction and separated in the y direction, wherein photons emitted in the positive x direction by the forward biased QCL return as echo photons in the negative x direction and are detected by the reverse biased APD. 
     
     
       10. The monolithic infrared transceiver of  claim 9 , further comprising:
 top and bottom metal coupling contacts that sandwich a third region of the heterostructure to form a m-i-m semiconductor between the first and second regions; said m-i-m semiconductor responsive to a bias to modify a refractive index in the third region optically separate the first and second regions. 
 
     
     
       11. The monolithic infrared transceiver of  claim 9 , further comprising:
 a third region of the heterostructure sandwiched between first and second mirrors in the z-direction and separated in the x-direction to form the laser cavity in the x direction and an n type doping layer above the laser cavity in which the heterostructure defines at least upper and lower energy states and a quantum well in each stage of a second quantum cascade laser (QCL), said third region exists side-by-side with the first region in a third waveguide separated in the y direction; 
 top and bottom coupling contacts that sandwich a fourth region of the heterostructure to form a m-i-m semiconductor between the first and third regions, said m-i-m semiconductor responsive to a bias to modify a refractive index in the fourth region to control lateral optical coupling between the first and third regions and the first and second QCLs. 
 
     
     
       12. The monolithic infrared transceiver of  claim 11 , wherein the heterostructure is comprised of a spintronic material, further comprising a source that applies a magnetic field in the z direction so that the optical coupling is controlled by both electrical and magnetic fields. 
     
     
       13. The monolithic infrared transceiver of  claim 8 , further comprising:
 a third region of the heterostructure sandwiched between first and second mirrors in the z-direction and separated in the x-direction to form the laser cavity and an n type doping layer above the laser cavity in which the heterostructure defines at least upper and lower energy states and a quantum well in each stage of a second quantum cascade laser (QCL), said third region exists end-to-end with the first region within the single waveguide, said third region responsive to the application of a forward bias such that the upper and lower energy states in each MOW stage are confined in the quantum wells to induce intersubband lasing transitions in each of the MOW stages to emit photons at a second wavelength in a positive x direction in a transmit mode to pump the first QCL. 
 
     
     
       14. The monolithic infrared transceiver of  claim 13 , wherein the second QCL is tuned to emit at one-half the energy separation of the upper and lower states of the first QCL to encourage two photon absorption. 
     
     
       15. The monolithic infrared transceiver of  claim 1 , wherein said first and second regions are a common region of the heterostructure in the x direction at the same position in the y direction, wherein said n and p type layers sandwich the common region, wherein said common region is responsive to application of a bias that switches between forward bias to emit light at the first IR wavelength and the reverse bias to detect light at the first IR wavelength. 
     
     
       16. The monolithic infrared transceiver of  claim 1 , further comprising:
 first and second top and bottom metal switching contacts (m) that form first and second m-i-m semiconductors in the heterostructure on opposite sides of the first region to provide the first and second mirrors that define the laser cavity, said m-i-m semiconductors responsive to the application of bias voltages to Q-switch the laser cavity and emit a pulse at the first IR wavelength. 
 
     
     
       17. The monolithic infrared transceiver of  claim 1 , further comprising:
 first and second top and bottom metal contacts (m) that form first and second m-i-m semiconductors on opposite sides of the first region in the heterostructure to provide the first and second mirrors that define the laser cavity, at least one said metal contact including a plurality of fingers spaced in the x direction to define a grating, said grating fingers responsive to an applied bias to change the length of the laser cavity in the x direction and tune the first IR wavelength. 
 
     
     
       18. The monolithic infrared transceiver of  claim 1 , further comprising:
 an optical low pass filter to remove echo photons whose energy is above the photon energy at the first IR wavelength; and 
 first and second top and bottom metal switching contacts (m) that form first and second m-i-m semiconductors on opposite sides of the second region in the heterostructure to provide the first and second mirrors that define the laser cavity, at least one said metal contact including a plurality of fingers spaced in the x direction to define a grating, said m-i-m semiconductors responsive to selective application of bias voltages to the grating fingers to modify the refractive index of the underlying heterostructure to form a high pass filter to remove echo photons below the first IR wavelength. 
 
     
     
       19. A monolithic infrared transceiver, comprising:
 a monolithic intrinsic (i) semiconductor heterostructure including N Multiple Quantum Well (MQW) stages stacked in a z-direction on a substrate, each stage having upper and lower energy states and a quantum well; 
 first and second regions of the heterostructure having the same N MQW stage composition and variation in thickness in the z direction for all positions in x and y directions; 
 a quantum cascade laser comprising said first region sandwiched between top and bottom first and second metal gratings oriented in the z-direction and separated in the x-direction to define mirror regions and a first n type doping layer to inject electrons into a first MQW stage, said mirror regions responsive to application of a bias to selected fingers of the gratings to change the refractive index and form mirrors to Q-switch a laser cavity and to tune the laser cavity a specified first IR wavelength, said first region responsive to application of a forward bias such that the upper and lower energy states in each MQW stage are confined in the quantum wells to induce intersubband lasing transitions in each of the MQW stages that contribute to a photon gain of the cavity to emit photons at the first IR wavelength in a positive x direction in a transmit mode; and 
 an avalanche photodiode (APD) comprising said second region of the heterostructure sandwiched between second p and n type doping layers in the z-direction that form a p-i-n semiconductor and between top and bottom third and fourth metal gratings oriented in the z-direction and separated in the x-direction to define minor regions, said mirror regions responsive to application of a bias to change the refractive index to tune a cavity to the first IR wavelength, 
 wherein said monolithic intrinsic heterostructure is configured such that a separation energy between the upper and lower energy states in the first region under forward bias equals or slightly exceeds a separation energy between the upper and lower energy states in the second region under reverse bias by a delta separation energy such that under reverse bias in each stage of the second region the lower energy state is confined to the quantum well and the upper energy state is near the top of the quantum well in energy and localized in the quantum well to spatially overlap with the lower energy state such that the upper energy states moves into an avalanche multiplication region of the APD to detect echo photons at the first IR wavelength incident in the negative x direction in a receive mode. 
 
     
     
       20. A monolithic infrared transceiver, comprising:
 a monolithic intrinsic (i) semiconductor heterostructure including N Multiple Quantum Well (MQW) stages in a z direction, each stage having upper and lower energy states and a quantum well; 
 a first region of the heterostructure in which under forward bias the upper and lower energy states are confined in the quantum well to form a quantum cascade laser (QCL) that emits photons in a positive x direction; and 
 a second region of the heterostructure, wherein said monolithic intrinsic heterostructure is configured such that a separation energy between the upper and lower energy states in the first region under forward bias equals or slightly exceeds a separation energy between the upper and lower energy states in the second region under reverse bias by a delta separation energy such that under reverse bias in each stage of the second region the lower energy state is confined in the quantum well and the upper energy state is near the top of the quantum well in energy and localized in the quantum well to spatially overlap the lower energy state such that the upper energy state moves into an avalanche multiplication region of an avalanche photodiode (APD) to detect echo photons incident in the negative x direction, 
 wherein said first and second regions have the same N MQW stage composition and variation in thickness in the z direction. 
 
     
     
       21. The monolithic infrared transceiver of  claim 20 , wherein the heterostructure comprises one or more of the following: each said MQW stage comprises alternating quantum well and thin barrier layers in which the thickness of the quantum well layers varies such that each MQW state is asymmetric about a center position of the stage in the z direction; an additional M MQW stages in the z-direction in the second region to reduce a voltage drop across each of N+M MQW stages in the second region under reverse bias; and the APD having a reverse polarity with respect to the QCL. 
     
     
       22. The monolithic infrared transceiver of  claim 21 , wherein in each stage of the second region under reverse bias the upper energy state is either confined just below the top of the quantum well or lies just above the quantum well in the continuum states.